This invention relates to a circuit device for driving an electromagnetically movable unit at a high speed with a single power source and, more particularly to a technique for electromagnetically driving a movable unit such as a fuel injection valve of an automotive engine with a single power source such as a DC 12 V battery at a high speed.
Most of the fuel injection valves high speed hydraulic valves for a conventional automotive engine have an electromagnetic coil and a resistor connected in series with the coil and are operated by switching a current flowing through the coil on or off in accordance with an input signal to thus switch the injection of fuel or the hydraulic pressure on or off with a single power source such as, for example, a DC 12 V battery.
Since the movement of the electromagnetically movable unit of the conventional circuit for driving the movable unit is conducted by a spring or a fuel pressure in one way, the force caused by the spring or the fuel pressure obstructs the electromagnetic force of the movable unit. Inasmuch as a resistor is further inserted in series with the electromagnetic coil of the conventional circuit, the movable unit does not sufficiently move with a high speed responsiveness with the input signal. In addition, if the resistance of the resistor is increased, the responsiveness of the movable unit decreases, and the resistance of the resistor cannot be considerably increased. Accordingly, the magnitude of the average current flowing in the electromagnetic coil of the movable unit becomes high, and the heat from the coil and the resistor becomes large and the coil uneconomically consumes a large amount of electric power.
Further, since the movable unit of the conventional circuit for driving the unit is driven by the electromagnetic force only in one way and driven by a mechanical force such as a spring or a fuel pressure in the return way, the movable unit such as a fuel injection valve is so constructed that the electromagnetic movable unit, and accordingly the permanent magnet unit, is of a solenoid valve type made of ferromagnetic material only. Consequently, the movable unit is attracted to a coil iron core unit to open the valve only if a current flows through the electromagnetic coil, even if the pole of the end of the movable unit of the coil iron core is either north N or S. However, when the valve seat is closed, the movable unit should interrupt the coil current and further return the movable unit by the force of the auxiliary spring and the fuel pressure. Thus, in order to accelerate the responsiveness of the valve with the movable unit in the conventional circuit, it must shorten the time required to return the movable unit with the result that the tension of the auxiliary spring should be strengthened. However, since the direction of the tension of the auxiliary spring acts reversely to that of the electromagnetic force for attracting the movable unit in the conventional circuit, the strengthened spring weakens the force for attracting the movable unit, causing the responsiveness for opening the valve seat to be decreased. As a consequence, it requires a large current and electric energy to produce a strong electromagnetic force considering the force of the auxiliary spring so as to obtain a quick responsiveness required of the movable unit.